Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. Wells are typically drilled using a drill bit attached to the lower end of a drill string. Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the bit, and may additionally carry drill cuttings from the borehole back to the surface.
In various oil and gas exploration operations, it may be beneficial to have information about the subsurface formations that are penetrated by a borehole. For example, certain formation evaluation schemes include measurement and analysis of the formation pressure and permeability. These measurements may be essential to predicting the production capacity and production lifetime of the subsurface formation.
While formation testing tools may be primarily used to collect fluid samples, other downhole tools may be used to collect core samples. For example, a coring tool may be used to obtain a core sample of the formation rock. The typical coring tool includes a hollow coring bit that is advanced into the formation to define a core sample which is then removed from the formation. The core sample may then be analyzed in the tool in the borehole or after being transported to the surface, such as to assess the reservoir storage capacity (porosity) and the permeability of the material that makes up the formation surrounding the borehole, the chemical and mineral composition of the fluids and mineral deposits contained in the pores of the formation, and/or the irreducible water content contained in the formation, among other things.
However, traditional coring tools are limited to obtaining sidewall core samples perpendicular to the longitudinal axis of the coring tool (or equivalently the wellbore axis), because the coring bit cannot be independently tilted and extended into the formation at an angle other than 90 degrees relative to the coring tool axis. Consequently, for laminated formations that exhibit anisotropy, where the intrinsic formation properties depend on a direction of measurement, a core sample extracted at a 90 degree angle must be subsequently cut along lines of anisotropy. The resulting sample is often not suitable for measurement of the desired formation property.